(1) Field of the Invention
This invention relates to a non-aqueous secondary cell comprising a negative electrode having lithium as an active material and a positive electrode having a lithium-including manganese oxide as an active material, especially to an improvement of the positive electrode.
(2) Description of the Prior Art
As an active material of the positive electrode of this kind of secondary cell, molybdenum trioxide, vanadium pentoxide and sulfide of titanium or niobium have been proposed and partially practically used into practical use.
As an active material of a positive electrode of a non-aqueous primary cell, manganese dioxide and carbon fluoride are known and have already been put into practical use. Especially, manganese dioxide is excellent in storage characteristics, abundant, and inexpensive.
Considering these features, manganese dioxide seems to be appropriate as an active material of the positive electrode of a non-aqueous secondary cell. However, manganese dioxide is poor in reversibility and charge/discharge cycle characteristic.
The inventors of this invention has disclosed a proposal for minimizing the above problems of manganese dioxide in U.S. Pat. No. 4,758,484 (Japanese Patent Publication Kokai No. 63-114064). According to the above patent, a lithium-including manganese oxide, namely, a manganese dioxide including Li.sub.2 MnO.sub.3, or a manganese oxide including lithium and having CuK.alpha. beam peaks at 2.theta.=22.degree., 31.5.degree., 37.degree., 42.degree. and 55.degree. is employed as a positive electrode active material. As a lithium-including manganese oxide, Li.sub.1-x Mn.sub.2 O.sub.4 (1&gt;x&gt;0) has been disclosed in U.S. Pat. No. 4,904,552.
According to the above proposal, the crystal lattice structure of the manganese oxide has reversibility with respect to the insertion and removal of lithium ion, which improves the cycle characteristic. However, other characteristics should also be improved to put it into practical use.
Although discharging manganese dioxide electrochemically has been proposed as a method of producing a lithium-including manganese oxide, it is more practical to mix a lithium salt and a manganese oxide and then to heat-treat the mixture. However, such a reaction between solid phases is performed non-uniformly if the manganese oxide mixed with the lithium salt has large-sized grains. As a result, a large portion of the above two chemicals remain unreacted, which prevents improvement in the cycle characteristic.